It is often desirable to produce sulfonic acids by reacting an aromatic compound with sulfuric acid, usually in the form of 20% oleum, a mixture of 20% SO.sub.3 and 80% H.sub.2 SO.sub.4. In many cases, this provides two or more aromatic sulfonic acids, at least one of which is preferentially oil soluble and at least one of which is preferentially water soluble. In King and Thielcke, U.S. Pat. No. 2,764,548, there is described a process for the production of dinonylnaphthalene monosulfonic acid. The salts of the monosulfonic acids are relatively highly soluble in oils, and form compositions having exceptional rust-inhibiting properties. It is also disclosed in the patent that the sulfonation of dinonylnaphthalene with sulfuric acid also produces the corresponding disulfonic acid, and this is described to be accumulated in an aqueous layer which is later separated and discarded. Because under common commercial conditions the predominant product is the monosulfonic acid, the disulfonic acid had been treated as a byproduct, and disposed of.
In fact, however, polyalkylaromatic polysulfonic acids, particularly alkylnaphthalene disulfonic acids and their derivatives are very effective in surfactant and catalyst applications. Dinonylnaphthlene disulfonic acid (DNNDSA) in particular is a commercially valuable catalyst in the preparation of coatings based on melamineformaldehyde resins. These applications have been disclosed in U.S. Pat. Nos. 3,979,478, L. V. Gallacher; and 4,200,729, L. J. Calbo, and others.
DNNDSA was reported in the above-mentioned U.S. Pat. No. 2,764,548 to be a by-product in the preparation of dinonylnaphthalene sulfonic acid. This patent describes the removal of the disulfonic acid and sulfuric acid from the monosulfonic acid product by batch-washing with water. Subsequently, in U.S. Pat. No. 3,957,859, Thielcke disclosed a continuous process for recovering DNNDSA from a hydrocarbon solution of mixed sulfonic acids by first extracting countercurrently with water in a multi-stage column and then extracting the aqueous solution countercurrently with a higher alkanol in a second multi-stage column. (As used herein, the term "DNNDSA" is intended to include not only dinonylnaphthalene disulfonic acid, but also mixtures of it with lesser amounts of monononylnaphthalene disulfonic acid and trinonylnaphthalene disulfonic acid.)
While these techniques are effective in recovering DNNDSA from crude sulfonic acid containing relatively low levels (ca. 1-10% based on the weight of dinonylnaphthalene monosulfonic acid), they are not suitable for high DNNDSA production rates starting with crude sulfonic acid streams containing high levels of DNNDSA.
In order to increase the level of DNNDSA in a crude sulfonic acid based on the sulfonation of dinonylnaphhthalene, it is necessary to increase the mole ratio of sulfonating agent to dinonylnaphthalene in order to increase the level of disulfonation in the product. This can be achieved in a number of ways with different reagents. For example, it can be done by treating a hydrocarbon solution of dinonylnaphthalene with 20% oleum, or, alternately, it may be done by contacting dinonylnaphthalene with a mixture of sulfur trioxide and air in a continuous reactor. This disclosure in no way limits the methods which can be used to prepare crude sulfonic acid mixtures containing relatively large amounts of DNNDSA.
It is an objective of the present invention to provide a novel means of separating high-purity DNNDSA from a crude sulfonic acid mixture comprising DNNDSA, dinonylnaphthalene monosulfonic acid, sulfuric acid, and hydrocarbon solvent.
It has now been discovered that when a crude sulfonic acid mixture containing DNNDSA, dinonylnaphthalene monosulfonic acid, a small amount of dissolved sulfuric acid and hydrocarbon solvent, is mixed with approximately one-half volume of water, and heated to a temperature of approximately 50.degree. C. to 90.degree. C., the mixture separates into three liquid phases: an organic upper layer containing monosulfonic acid and solvent, a middle layer containing sulfuric acid in water, and a lower layer containing primarily DNNDSA and water. Surprisingly, in the preferred embodiment, the lower DNNDSA layer contains approximately one-half or more of the total DNNDSA in the feed at a concentration of approximately 50% in water. Very little sulfuric acid or hydrocarbon solvent are present in this lower layer. Thus, the lower layer can be readily separated and processed directly to yield a high quality DNNDSA concentrate or product.
It has further been discovered that the process is widely applicable to the production of other polyalkylaromatic polysulfonic acids, such as didodecylnaphthalene disulfonic acid and these valuable products are also provided in high purity and high yields.